The present invention relates to a method and apparatus for controlling a percussion mechanism which has a striking piston that performs back and forth movements under the influence of hydraulic driving medium furnished by a conveying unit, and more particularly to a method and apparatus for adapting the operational behavior of the percussion mechanism to the hardness of material that is being pounded (for the purpose, for example, of crushing the material) by the percussion mechanism. The percussion mechanism employed may (or may not) be one having an adjustable stroke.
Hydraulically operated percussion mechanisms form an operational chain with a conveying unit such as a hydraulic pump on one side and a tool (particularly a chisel) on the other side for crushing material. A stream of energy flows through this chain in each direction. The primary stream of energy is directed from the conveying unit through the percussion mechanism and the tool onto the material to be crushed (such as rock or stone). A secondary stream of energy may be reflected by the material being crushed and act through the percussion mechanism in the direction toward the conveying unit. The characteristics of the material being crushed and of the tool are expressed by a reflection factor R, which is an indirectly measurable value and is defined by the ratio of the primary striking energy to the reflected striking energy. The magnitude of the reflection factor may influence the energy balance and the characteristics (that is, the ratio of striking rate to the energy of each individual stroke) of the percussion mechanism.
To attain the greatest possible crushing performance, it would be desirable to adapt the operational behavior of a percussion mechanism in such a way that, with increasing hardness of the material (that is, with increasing magnitude of the reflection factor), the energy of each individual stroke increases. A reflection or rebound may occur if the energy in an individual stroke is too low. Such reflections are undesirable, since stresses on the tool and the percussion mechanism are high but the crushing performance is poor. In order to limit the stresses on the percussion mechanism, care should be taken that the number of striking piston strokes does not increase with increasing reflection of energy. Furthermore, for a percussion mechanism of the type having an adjustable stroke, care should be taken that an increase in the energy of each individual stroke due to an adjustment of the magnitude of the stroke is accompanied by a suitable reduction of the striking rate.
Various methods have been proposed to adapt the operational behavior of a percussion mechanism to the hardness of the material being crushed. These methods utilize the varying period of dwell of the striking piston in the vicinity of its theoretical striking position (see EP-B1-0,214,064, corresponding to U.S. Pat. No. 4,899,836 and EP-B1-0,256,955, corresponding to U.S. Pat. No. 4,800,797) or are based on electrical measurements of the vibrations in drill rods and derive therefrom an influence on the position of the advancing unit (see DE-A1-3,518,370, corresponding to U.S. Pat. No. 4,671,366). In the case of the first-mentioned method, a pressure fluctuation is initiated within the percussion mechanism as a function of the period of dwell and is utilized directly as a control value which adjusts the impact velocity and the striking rate as a function of the hardness of the material being crushed. An undesirable recoil of the striking piston due to insufficient energy of an individual stroke can be avoided at least substantially by means of a control change which influences the magnitude of the stroke of the striking piston (see DE-C3-2,658,455, corresponding to British patent 1,584,810).
Some of the above-mentioned proposals for a solution to the problem of adapting the operational behavior of a percussion mechanism to the hardness of the material being crushed are not suitable for rough working conditions, are susceptible to malfunction but are not easy to repair, and/or are sensitive to changes in the temperature and changes in viscosity of the hydraulic driving medium. This applies, in particular, for controls whose essential components are integrated in the percussion mechanism.
A prior art percussion mechanism which employs energy recovery when the tool rebounds due to the reflection factor is disclosed in European Patent Office Publication 0,183,093, corresponding to U.S. Pat. No. 4,646,854, and will be described below with reference to FIGS. 8-11.
Percussion mechanism 101 includes an operating cylinder 102 in which a percussion piston 103 is movably guided. At its lower end, percussion mechanism 101 is provided with a bit 104 which is held in a known manner so as to be able to slide a certain amount. The end of operating cylinder 102 facing bit 104 is connected, via a feed or pressure conduit 105, with a source of hydraulic driving medium (hydraulic fluid) at a pressure P.sub.0. Facing the mouth of the inlet of pressure conduit 105 in operating cylinder 102, there is provided an annular groove 106. Percussion mechanism 101 is further equipped with an accumulator 108 which is connected with pressure conduit 105. The accumulator 108 may be designed as described, e.g., in U.S. Pat. No. 3,322,210 to Arndt.
Percussion mechanism 101 further includes a control valve 110 having a control cylinder 111 and a sleeve-like control slide 112 movable therein. Control cylinder 111 has three cylindrical parts, 111a, 111b, 111c (see FIG. 10), with center part 111b having the largest diameter, lower part 111c having the smallest diameter, and upper part 111a having a diameter which lies between the other two diameters. The front end of the lower cylindrical part 111c of control cylinder 111 is connected with pressure conduit 105 via branch conduit 113. The upper end of control cylinder 111 is connected, via an annular groove 115 and a conduit 116, with a return flow conduit 117 (hereinafter abbreviated as "return conduit") which leads to an essentially pressureless connection (symbol: P.sub.T) for releasing to hydraulic fluid.
The lower part 111c of control cylinder 111 is additionally connected, via a lateral conduit 118, with the upper part of operating cylinder 102, with an annular groove 119 being provided in control cylinder 111 toward the opening of conduit 118 and an annular groove 120 in operating cylinder 102. In the vicinity of groove 119, control cylinder 111 is connected via annular groove 122 directly with return conduit 117.
Between grooves 106 and 120 in operating cylinder 102, a further annular groove 123 is provided which is connected with return conduit 117 via two conduits 124 and 125. Between grooves 106 and 123, operating cylinder 102 is provided with a control annular groove 126 through which operating cylinder 102 is connected with control cylinder 111 via a control channel or a control conduit 127.
Control conduit 127 is formed by the two conduits or conduit sections 127a and 127b. In control cylinder 111, control conduit 127 opens into an annular groove 129 disposed between grooves 115 and 122. The four grooves 115, 129, 122, and 119 of control cylinder 111 define its three cylindrical parts as follows: the upper cylindrical part 111a lies above groove 115, the center cylindrical part 111b is delimited by the two grooves 115 and 129, and the lower cylindrical part 111c extends from the lower edge of groove 129 to below groove 119. Groove 122 lies within lower cylindrical part 111c.
The two conduit sections 127a and 127b are connected together by means of a holding valve 130 provided with three connections, with the third connection being connected, via a conduit section 131 and conduit 125, with return conduit 117.
Percussion piston 103 has a tapered, front or lower section 132 and a rear or upper section 133, with the diameter of front section 132 being greater than the diameter of rear section 133. Sections 132 and 133, respectively, are provided with a small annular face 134 and a large annular face 135, respectively. Between the two annular faces 134 and 135 there is disposed a recess or circumferential groove 136, forming two piston collars 137 and 138, respectively. When ready for Operation, the small annular face 134 is constantly charged with a hydraulic fluid such as oil through pressure conduit 105. With its end adjacent circumferential groove 136, piston collar 137 forms an upper control edge 139 and with its end formed by the smaller annular face 134, it forms a lower control edge 140.
At its lower end facing branch conduit 113, control slide 112 is equipped with a first cylindrical section 141 having a first frontal face 142 and at its opposite end, it has a second cylindrical section 143 having a second frontal face 144. The diameter of first section 141 is smaller than the diameter of second section 143, so that first frontal face 142 also is smaller than second frontal face 144. In the operational state, both frontal faces 142 and 144 are constantly under pressure so that, via these faces, a downwardly directed force constantly acts on control slide 112, with this force resulting from the difference between surface areas (A.sub.44 -A.sub.42) multiplied by the operating pressure P.sub.0. Between sections 141 and 143 there is a piston collar 145 having an annular or control face 147 which is associated with first section 141 and a correspondingly smaller annular face 148 which is associated with second section 143. The space between groove 115 of control cylinder 111 and the second cylindrical section 143 of control slide 112 is always connected with return conduit 117.
The first section 141 of control slide 112 is guided in lower cylindrical part 111c and the second section 143 is guided in the upper cylindrical part 111a of control cylinder 111. Piston collar 145 slides in the center, cylindrical portion 111b of control cylinder 111.
The first cylindrical section 141 has a circumferential groove 149 and--between it and control face 147 --a small radial auxiliary bore 151. The length of circumferential groove 149 is greater than the distance between the two grooves 119 and 122 in control cylinder 111.
In FIG. 9, holding valve 130 is shown to an enlarged scale. It is comprised of a hollow cylindrical cavity or bore 153 and a valve body in the form of a sphere 154. Cylindrical cavity 153 is provided with a valve seat 155 at its frontal face which is connected, via conduit section 131, with return conduit 117 and, at its other frontal face, which is connected with conduit section 127a, cylindrical cavity 153 is provided with a valve seat 156. Conduit section 127b of control section 127 opens radially into cylindrical cavity 153. If sphere 154 rests against valve seat 155, the center axis of conduit section 127b goes through the half of sphere 154 facing this valve seat. The diameter of sphere 154 is smaller than the diameter of cylindrical cavity 153 so that the hydraulic fluid is able to flow through the gap 158 formed by sphere 154 and bore 153.
The operation of percussion mechanism 101 will be described below for the case where the energy applied to bit 104 by percussion piston 103 is transferred completely to the pulverized material and is not reflected. FIG. 8 shows percussion mechanism 101 during the operating stroke of percussion piston 103. Above the large annular face 135, cylinder 102 is connected via conduit 118 and branch conduit 13 with pressure conduit 105. Percussion piston 103 is thus accelerated toward bit 104 by a force which is the result of the surface area difference (A.sub.35 -A.sub.34) multiplied by the operating pressure P.sub.0. During the striking or operating stroke, control slide 112 is in the upper position facing away from high pressure conduit 105 and branch conduit 113, respectively. Hydraulic fluid can flow through auxiliary bore 151 into the area below control face 147 and maintains the pressure existing there (conduit section 127a is covered by piston collar 137 and conduit 131 leading to return conduit 117 is covered by sphere 154 resting against the left valve seat 155). Since annular face 148 is without pressure and control face 147 is larger than the downwardly acting partial surface or difference between surface areas (A.sub.44 -A.sub.42), control slide 112 remains in its upper position.
Shortly before piston 103 abuts on bit 104, the upper control edge 139 reaches control groove 126. Circumferential groove 136 connects conduit section 127a--and thus the entire control conduit 127--via conduit 124 with return conduit 117, so that the area below control face 147 becomes free of pressure. Thus the force composed of the partial area or surface area difference (A.sub.44 -A.sub.42) is the exclusive force acting on control slide 112 and, during a valve or control period t.sub.st, presses control slide 112 from its upper position (illustrated in FIG. 8) to its lower position (illustrated in FIG. 10). This causes the (pressure-free) hydraulic fluid to be pressed through conduit section 127b which, when it enters cylindrical cavity 153 of holding valve 130, impinges on the left portion of sphere 154 and flows through the gap 158 formed by sphere 154 and cavity 153 and through conduit section 127a to return conduit 117. The pressure difference appearing at gap 158 drives sphere 154 to the right toward valve seat 156. For a short time, the hydraulic fluid which flows out through conduit section 127b, can also flow out through conduit section 131 and through conduit section 127a as well as through conduit 124 to conduit 125 and thus to return conduit 117. If sphere 154 rests against valve seat 156, and conduit section 127a and conduit 124 are thus blocked for the hydraulic fluid flowing out of conduit section 127b, the hydraulic fluid that is displaced from control face 147 and that is being fed in through auxiliary bore 151 can continue to flow out, against a slight drop in pressure in conduit section 131, to conduit 125 and thus to return conduit 117.
While control slide 112 is being moved into its lower position, auxiliary bore 151, on the one hand, is covered by that part of control cylinder 111 which is disposed between grooves 129 and 122, and circumferential groove 149, on the other hand, connects annular face 135 of piston 103 via grooves 122 and 119 as well as conduit 118 with return conduit 117 (see FIG. 10). The upper, large annular face 135 is thus relieved and percussion piston 103 is accelerated upwardly for its return stroke by the force acting on the lower, small annular face 134. Toward the end of the return stroke, the lower control edge 140 in the form of the smaller annular face 134 reaches control groove 126 and thus reestablishes a connection between pressure conduit 105 and conduit section 127a of control conduit 127. Sphere 154 is pressed toward the left valve seat 155 (compare FIG. 11) and the entire control conduit 127 as well as the area below control face 147 is charged with pressure so that control slide 112 is moved back into its upper position in which it reconnects, via connection conduit 113, conduit 118--which leads to operating cylinder 102--with pressure conduit 105 and initiates a new operating stroke.
For the case that the energy transmitted from percussion piston 103 to bit 104 is at least partially reflected, the reflected pulse suddenly accelerates piston 103 in the direction of the return stroke so that the time between the opening of control groove 126 via circumferential groove 136 and conduit 124 to return conduit 117 during the downward stroke of piston 103 and reclosing of control groove 126 by the upper control edge 139 is too short to bring control slide 112 into its lower position.
A small downward movement of control slide 112 may possibly be initiated but, when control groove 126 is prematurely closed because of the reflection, so that hydraulic fluid is prevented from flowing from conduit section 127a to conduit 124, this initial downward movement is cancelled out because auxiliary bore 151 communicates hydraulic fluid into the area below control face 147.
The short time between the opening of control groove 126 by the upper control edge 139 and the premature reclosing of groove 126 when piston 103 jumps back is, however, sufficient to lift sphere 154 from the left valve seat 155 and move it to the right. But this connects conduit section 127b with return conduit 117 via conduit sections 131 and 125. This connection remains intact even if control groove 126 is closed again. Control slide 112 thus performs its change of position in the same manner and during nearly the same valve or control period t.sub.st as if percussion piston 103 were to release all of its energy without reflection to bit 104 and remain in the impact position S-S.
Within the control or valve period t.sub.st, i.e. within the period running from the connection of the annular control groove 126 by the circumferential recess 136 via the conduit 124 to the pressureless return conduit 117 to that moment, when the control slide 112 has reached its lower position, the pressurized fluid ahead of the large annular area 135 within the operating cylinder 102 and being pushed upwards by the percussion piston 103 (which in turn is reflected or moved upwards by bit 104) can flow off through the conduits 113 and 118 into the accumulator 108. The control period t.sub.st is structurally selected in such a manner that, in the case of a reflection of the striking energy, the pressurized fluid being pushed upwards can flow off into the accumulator 108 essentially completely; that means that this energy is thereby additionally available at the following stroke. For that purpose the control or valve period t.sub.st may be influenced or modified by the areas A.sub.47 and A.sub.48 of the control face 147 and of the annular face 148, respectively, whereby the control period t.sub.st is inversely proportional to the square root of the area difference (A.sub.47 -A.sub.48). Further elements for modifying the control period t.sub.st are the stroke s.sub.12 and the mass m.sub.12 of the control slide 112 as well as the operating pressure P.sub.0, whereby the control period t.sub.st is linear or proportional to the root of the stroke s.sub.12 and to the root of the mass m.sub.12 and reversely proportional to the root of the operating pressure P.sub.0. For further modifying the control period t.sub.st the operating pressure may be reduced by an adjustable throttle valve as described, e.g., in British Patent 1,584,810.
The hydraulic fluid flowing into pressure reservoir 108 is available for the next percussion stroke and has the same effect as an increase in the conveyed quantity Q.sub.0 of fluid. Since the striking rate z of percussion piston 103 is proportional to the conveyed quantity Q.sub.0, the availability or recovery, respectively, of the reflected energy also increases the striking rate z and thus the entire output of percussion mechanism 101.
In modification of the described embodiment, control slide 112 can also be charged by a pressure spring in the direction towards its lower position, as disclosed, for example, in European Patent Application A1 0,070,246.
The operation of percussion mechanism 101 as described above can be summarized as follows: When piston 103 is in its upper position (the position of piston 103 in FIG. 10), an upward force is exerted on control slide 112 because the region beneath control face 147 is effectively exposed to pressure conduit 105. However when control slide 112 is in its upper position (the position of slide 112 in FIG. 8), a downward force is exerted on piston 103 because conduit 118 is effectively connected to connected to pressure conduit 105. The result is that piston 103 begins moving downward, while slide 112 remains biassed in its upper position. When piston 103 is near its lower position (not illustrated), however, its recess 136 connects conduit section 127a to conduit 124. This effectively exposes the region beneath face 147 to return conduit 117, despite a slight flow of fluid through bore 151, so that slide 112 begins moving toward its lower position (the position of slide 112 illustrated in FIG. 10). When it reaches its lower position slide 112 effectively disconnects conduit 118 from pressure conduit 105 and exposes it instead to return conduit 117, so that large face 135 gets pressure-free and the operating pressure P.sub.0 on face 134 of piston 103 begins moving piston 103 back to its upper position to begin the cycle anew. This operation occurs even if piston 103 rebounds when it strikes bit 104, so that conduit section 127 is disconnected from conduit 124 before slide 112 reaches its lower position. The reason for this is that sphere 154 is lifted from valve seat 155 as soon as conduit section 127a is connected to conduit 124 on the downward stroke of piston 103, thereby effectively connecting the region beneath face 147 to return conduit 117 until piston 103 regains its upper position.
Further information concerning percussion mechanism 101 can be found in U.S. Pat. No. 4,646,854, which is incorporated herein by reference.